The present invention relates to a method for a more efficient use of a combustion engine in a vehicle, by, during certain vehicle conditions, automatically controlling engine rotational speed in a predetermined way.
The present invention also relates to a computer program and computer program product both to be used with a computer for executing said method.
In vehicles equipped with an automatic transmission a situation can sometimes occur in uphill drive where a transmission control unit estimates that an upshift to a new gear will not be possible due to that the engine of the vehicle will not be able to accelerate the vehicle to a sufficiently high vehicle speed so that the engine will manage to drive the vehicle with said new gear engaged. An alternative to the estimation, that the engine of the vehicle will not be able to accelerate the vehicle to a sufficiently high vehicle speed, is to simply register that the engine rotational speed increase has stopped before reaching the right engine rotational speed for upshift. This usually leads to the engine being stuck or “hanging” at relatively high engine rotational speeds without the transmission being able to perform an upshift. A typical example for a heavy duty truck could be that the engine must increase its speed to above 1850 rpm to be able to after an upshift land on a rotational speed with the new gear engaged that gives the engine the possibility to produce enough torque to at least be able to maintain current vehicle speed. In order to gain a better margin it could be necessary to increase the engine rotational speed to even higher rotational speeds, such as 1900 to 1950 rpm, before a gear upshift to the new gear is initiated. At these relatively high engine rotational speeds the efficiency of the engine is relatively low, and having the engine stuck at these relatively high engine rotational speeds contributes to an increased fuel consumption.
JP08-295154 discloses a cruise control in a vehicle which adapts the vehicle speed (decreases it) in for example an uphill so that the engine rotational speed arrives in a more efficient working area.
It is desirable to control the engine rotational speed in a better way in relation to a planned upshift.
The method according to an aspect of the invention is a method for a more efficient use of a combustion engine in a vehicle during driving of said vehicle, said vehicle further comprising an automatic step geared transmission for automatic gear ratio adaptation of the gear ratio between the engine rotational speed and rotational speed of driving wheels of the vehicle. Said method comprises the steps of: —sensing current engine rotational speed and engine rotational speed increase, —estimating necessary minimum upshift engine rotational speed for a coming gear upshift; and where said method is further characterized in the steps of:                registering that the engine rotational speed stops increasing without reaching said minimum upshift engine rotational speed, and where said engine rotational speed stops increasing relatively close to a maximum engine rotational speed where engine efficiency is relatively low and,        automatically controlling engine output torque in order to limit the engine rotational speed to a first predetermined engine speed where engine efficiency is relatively high.        
The advantage with the method according to an aspect of the invention is that fuel will be saved since unnecessary driving at inefficient engine rotational speeds will be decreased.
According to one embodiment of the method according to an aspect of the invention said step of registering that the engine rotational speed stops increasing without reaching said minimum upshift engine rotational speed is performed in advance before reaching said relatively high engine rotational speed where efficiency is relatively low. The advantage with this embodiment is that the driver will not experience any sudden power loss and vehicle speed decrease.
According to a further embodiment of the method according to an aspect of the invention, said registering step performed in advance is done through an estimation based on present prevailing vehicle conditions. In a further embodiment of the method according to the invention, said estimation is based on present prevailing vehicle conditions and vehicle conditions estimated likely to occur in near future of the vehicle. The advantage with this embodiment is that the activation and use of said automatic controlling of the engine torque can be further optimized and even more fuel can be saved.
According to one embodiment of the method according to an aspect of the invention said automatic controlling of the engine torque can only be active when a cruise control arranged in the vehicle is active. This embodiment allows the driver to have more of a full manual control when driving with the cruise control inactivated.
According to a further embodiment of the method according to an aspect of the invention, said automatic controlling of the engine torque can only be active when a first vehicle driving mode is activated, which first driving mode differs from a second driving mode of said vehicle. This first driving mode can for example be an economy mode that can be manually selected and engaged by the driver.
According to a further embodiment of the method according to an aspect of the invention, said automatic controlling of the engine torque is inactivated when a driver of the vehicle demands full power to the driving wheels. This gives the driver the possibility to override said automatic controlling in an easy and intuitive way. Thus, for example depressing an accelerator pedal to the floor will give a down shift and vehicle acceleration.
According to a further embodiment of the method according to an aspect of the invention, said automatic controlling of the engine torque is inactivated when a predetermined vehicle condition is registered. This predetermined vehicle condition can for example be a decrease of vehicle travel resistance to under a predetermined level. Thus, when the incline of an uphill slope decreases so much that the vehicle travel resistance is below said predetermined value, which decides if said upshift will be possible to perform or not, then the control unit will allow engine rotational speed increase to the upshift engine rotational speed. In a further embodiment said automatic controlling of the engine torque is activated when said vehicle travel resistance has increased to above a second predetermined level. Said first and second level can have the same value or differ from each other.
According to a further embodiment of the method according to an aspect of the invention, said automatic controlling of the engine torque is independent of if a driver of the vehicle demands maximum engine output torque or not. Thus, a full automation will minimize the time the engine spends in the inefficient rotational speed area.